Lithium ion secondary batteries, which feature small size and large capacity, have been widely used as power supplies for electronic devices such as mobile phones and notebook computers and have contributed to enhancing convenience of mobile IT devices. In recent years, larger-scale applications, such as power sources for automobiles and electrical storage devices for smart grids, have attracted attention.
Demand for lithium ion secondary batteries has increased, and as batteries are used in various fields, further higher energy density of batteries is required. As a positive electrode active material of a lithium secondary battery (lithium ion secondary battery), use of materials of having a layered structure such as LiCoO2, LiNiO2, LiNi1/3Co1/3Mn1/3O2 may be exemplified. A feature of these materials is that an average discharge potential of 3.7 V to 4.0 V versus Li can be obtained with a discharge capacity of 150 mAh/g or more. Since these materials can increase the energy density of the battery, they have been widely used as small portable batteries. However, Ni and Co are disadvantageous in the case of forming a large-sized battery in terms of high cost of materials.
As another positive electrode material having a layered structure which has high energy density, examples of using of Li2MnO3—LiMO2(M=Mn, Co, Ni) and a Li rich layered compound for example, represented by Li(NixLi(1/3-2x/3)Mn(2/3-x/3))O2 as a positive electrode active material have been reported in recent years. These materials have an advantage that the charge termination voltage is about 4.8 V, which is higher than that of the conventional positive electrode active material having a layered structure, and energy density is high. When these Li rich layered compounds are used as positive electrode active materials under a high temperature such as 45° C. or low charge and discharge rate such as 0.025 C (herein, when a battery having a certain capacity is discharged with a constant current, the current which finishes discharge in exactly one hour is referred to as 1 C), a high capacity of 200 mAh/g or more can be obtained. However, the problem was found after the investigation, that under high charge-discharge rate such as 1 C or more at a temperature of 20° C., capacity is little exhibited. This is considered to be due to the low ionic conductivity and electronic conductivity of the Li rich layered compound, and thus there is room for improvement as a characteristic of the battery to be normally used.
On the other hand, another candidate includes a material of spinel structure typified by LiMn2O4. Discharge capacity is about 110 mAh/g, the average discharge voltage is about 4.0 V and the energy density is smaller than that of the material of the layered structure, but it comprises Mn as a main component, has an advantage in terms of cost, and has the advantage that the safety of the battery can be easily enhanced due to high thermal stability during charge.
It is also considered that LiNi0.5Mn1.5O4 having the same structure as LiMn2O4 and having high charge-discharge potential. Since discharge capacity is about 135 mAh/g and average discharge potential exhibits high of about 4.6V or more versus Li, the equivalent of LiCoO2 and the like in terms of energy density can be obtained. Because of a spinel structure, the thermal stability during charging is as high as LiMn2O4. Further, as for a spinel structure such as LiNi0.5Mn1.5O4, an energy density of 90% or more can be obtained even at a low temperature such as −20° C. Since the positive electrode active material of spinel structure has high ionic conductivity, it can be used over a wide range of temperature and charge-discharge rate. For these reasons, LiNi0.5Mn1.5O4 is promising as a positive electrode material.
As a method of increasing the initial charging capacity of the lithium-manganese composite oxide having a spinel structure which has a smaller initial charge capacity than that of a positive electrode material having a layered structure such as LiCoO2, a method of previously doping Li into the positive electrode active material is exemplified. Patent Document 1 discloses a technology for increasing the capacity of a battery by using Li1+XMn2O4 (X>0) obtained by doping Li into a lithium manganese composite oxide as a positive electrode active material and causing phase transition to LiMn2O4 during charge. Examples of the method of doping Li include a method of electrochemically doping Li as disclosed in Patent Document 2 and a method of obtaining by chemical reduction of LiMn2O4 as disclosed in Patent Document 3. Patent Documents 4 to 7 disclose examples of using a positive electrode active material comprising a tetragonal lithium-manganese composite oxide represented by Li1+XMn2O4 (X>0) or the like and a cubic lithium-manganese composite oxide represented by LiMn2O4 or the like, in order to suppress decrease in capacity by compensating for Li which is irreversibly taken into the negative electrode at the initial charge.
As another technique, there are some examples of techniques in which positive electrode active materials are mixed and used. For example, Patent Document 8 discloses a nonaqueous electrolyte secondary battery in which a lithium-manganese composite oxide represented by LiMn2O4 or the like is used as a positive electrode active material and a lithium-nickel composite oxide represented by LiNi1-xCoxO2 (x is generally 0.1 to 0.4) or the like is mixed in the positive electrode. In such a mixture, the combination of positive electrode active materials is important. For example, LiNi1-xCoO2 has a disadvantage that its crystal structure is unstable in an overcharged state, but has an advantage that it has high capacity. On the other hand, LiMn2O4 has an advantage that safety during charging is high but the capacity is slightly small, and such disadvantages are compensated by the mixture. It is also shown that addition of LiNi1-xCoxO2 as a hydrogen ion trapping agent reduces Mn elution and provides a high reliability.
Patent Document 9 discloses a positive electrode for lithium secondary batteries comprising a composite oxide having a spinel structure in which charge and discharge is performed at a high voltage of 4.5 V or more represented by Li[M1mM22-m]O4 such as Li[Ni0.5Mn1.5]O4 and a composite oxide having a layered structure containing Mn such as Li[Ni0.33Li0.1Mn0.57]O2. By comprising a composite oxide having a spinel structure such as LiNi0.5Mn1.5O4 which can be used at high charge-discharge rate and over a wide range of temperature, the disadvantage of a positive electrode active material having a layered structure containing Mn whose ion conductivity is low is compensated. Further, by using a composite oxide of a layered structure having a high energy density mixed with a composite oxide having a spinel structure such as LiNi0.5Mn1.5O4, high energy density of a battery over a wide range of charge-discharge rates and temperatures is achieved.
As a negative electrode material, carbon materials are now mainly used. Meanwhile, negative electrode materials such as Si-based and Sn-based materials have been studied. Since these materials exhibit a large charge-discharge capacity as compared with carbon materials, higher energy density can be expected. However, there has been the problem that the charge-discharge efficiency at initial charge-discharge is low as compared with positive electrode active materials. For this reason, irreversible capacity is large at the initial charge-discharge, and most of Li in the positive electrode is consumed by irreversible capacity.
As for lithium ion batteries, although the above-mentioned materials are used for the positive electrode, there has been no positive electrode sufficiently satisfying rate and cycle characteristics while sufficiently compensating Li of an irreversible capacity of a negative electrode and exhibiting high discharge energy. In practical batteries, there are always requirements for rapid development of portable electronic devices and achieving higher energy for such as longer travel distance and smaller weight of loaded batteries of electric cars, and thus there has been a problem about increasing the capacity of the secondary battery.